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To guide the selection of a suitable fluorinated ether (FE) co-solvent for lithium metal batteries, it is crucial to understand the relationship between the organic structures of the FEs and the electrochemical performance of an FE-containing electrolyte. In this work, 1,1,2,2-tetrafluoro-1-(2,2,2-trifluoroethoxy)ethane (FEE), 1,1,2,2-tetrafluoro-3-(1,1,2,2-tetrafluoroethoxy)propane (TTE), and 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane (OFDEE) were chosen as representative FE co-solvents because of their distinct structural properties. The structure-activity relationship between the FEs and the electrochemical performance of Li||LiNi

A complementary electrolyte system with 0.8 M lithium bis(fluorosulfonylimide) (LiFSI) salt and 2 wt % lithium perchlorate (LiClO4) additive in fluoroethylene carbonate (FEC)/ethyl methyl carbonate (EMC) solution enables not only stable cycling of lithium metal batteries (LMBs) with practical loading ( 4 mAh/cm2) but also outstanding degradation stability toward the end of cycle life when compared to the conventional electrolyte. Although the use of LiFSI salt can increase the electrolyte conductivity and lengthen the cycle life of LMBs, the aged lithium anode morphology formed by the sacrificial decomposition of LiFSI is highly porous, leading to an abrupt cell capacity drop toward the end of cycling. Moreover, the inability to stop aluminum corrosion by the LiFSI-based electrolyte also causes cracking of the cathode tab during prolonged cycling. It is observed that a highly porous aged lithium consumed electrolyte at a higher rate, leading to the dry-out of electrolyte solvents. On the contrary, dense aged lithium anode morphology increased the localized current applied on the lithium, causing the formation of lithium dendrite. Thus, porosity control is the key to enhance battery performance. In this complementary system, LiClO4 was introduced as an advanced additive to not only improve the capacity retention rate but also mitigate the abrupt capacity drop toward the end of cycle life because LiClO4 acted as a pore astringent reducing the porosity of the aged lithium metal anode to the desired level. Moreover, the addition of LiClO4 can also suppress the Al corrosion, allowing stable high-voltage cycling of LMBs. The synergistic effect of combining LiFSI salt and a LiClO4 additive leads to an electrolyte system that can facilitate the application of high-energy LMBs with practical electrode loading.

A new class of fluorinated sulfones, β-fluorinated sulfones, were designed and synthesized as electrolyte solvents for high voltage lithium-ion batteries. While the oxidation potential of β-fluorinated sulfones is slightly lower than that of α-fluorinated sulfones, it is still significantly higher than the oxidation potential of regular sulfones, which already possess fairly high anodic stability. However, β-fluorinated sulfones exhibit a significant decrease in reduction potential compared to α-fluorinated sulfones, rendering them more stable towards graphite anodes. Moreover, the reduced lithium solvating power of β-fluorinated sulfones compared to regular sulfones mitigates the transition metal dissolution of cathodes. Taken together, these middle ground properties of β-fluorinated sulfone-based electrolytes enable the very stable long-term cycling of graphite‖LiNi0.6Co0.2Mn0.2O2 full cells. The outstanding performance of β-fluorinated sulfones designed by applying the “golden middle way” paves a new path for the development of an effective electrolyte system.

Despite the exceptionally high energy density of lithium metal anodes, the practical application of lithium-metal batteries (LMBs) is still impeded by the instability of the interphase between the lithium metal and the electrolyte. To formulate a functional electrolyte system that can stabilize the lithium-metal anode, the solvation behavior of the solvent molecules must be understood because the electrochemical properties of a solvent can be heavily influenced by its solvation status. We unambiguously demonstrated the solvation rule for the solid-electrolyte interphase (SEI) enabler in an electrolyte system. In this study, fluoroethylene carbonate was used as the SEI enabler due to its ability to form a robust SEI on the lithium metal surface, allowing relatively stable LMB cycling. The results revealed that the solvation number of fluoroethylene carbonate must be ≥1 to ensure the formation of a stable SEI in which the sacrificial reduction of the SEI enabler subsequently leads to the stable cycling of LMBs.

Hydrofluoroethers (HFEs) have been adopted widely as electrolyte cosolvents for battery systems because of their unique low solvating behavior. The electrolyte is currently utilized in lithium-ion, lithium-sulfur, lithium-air, and sodium-ion batteries. By evaluating the relative solvating power of different HFEs with distinct structural features, and considering the shuttle factor displayed by electrolytes that employ HFE cosolvents, we have established the quantitative structure-activity relationship between the organic structure and the electrochemical performance of the HFEs. Moreover, we have established the linear free-energy relationship between the structural properties of the electrolyte cosolvents and the polysulfide shuttle effect in lithium-sulfur batteries. These findings provide valuable mechanistic insight into the polysulfide shuttle effect in lithium-sulfur batteries, and are instructive when it comes to selecting the most suitable HFE electrolyte cosolvent for different battery systems.

Discovering new electrolyte additives for specific purposes has been a long-run demand for lithium batteries. With the help of the current understanding of failure mechanisms of lithium batteries, it is no longer impossible to have a rational design, instead of random trials, of electrolyte additives for lithium batteries. More excitingly, we have successfully discovered several new electrolyte additives based on chemistry knowledge. Standard characterization tools, such as NMR, MS, SEM, and XPS, are enough to verify the proposed design rules for these electrolyte additives. Phosphorus pentoxide (P2O5) is investigated as an acid scavenger to remove the acidic impurities in a commercial organic carbonate electrolyte for lithium batteries. We revealed that commercial electrolytes can be significantly improved for Li metal batteries via a simple modification with P2O5. The results indicate that adding P2O5 removes HF from the electrolyte and generates POxFy species which form a favorable SEI. The new electrolyte is also capable of eliminating transition metal leaching and particle cracking of a Li-ion cathode. Acrylonitrile (AN) is another effective additive in carbonate-based electrolytes. It is verified as a solid electrolyte interphase (SEI) enabler resulting in a uniform and dense lithium (Li) deposition. Our electrochemical and spectroscopic study reveals that AN is cathodically electropolymerized on the Li surface prior to the electrochemical decomposition of the electrolyte during Li deposition. The resultant polyacrylonitrile artificial solid electrolyte interphase enables uniform nucleation and growth of Li deposition with significantly reduced side reactions. Several oxygen scavengers will also be introduced in this poster. Based on NMR and MS results, these oxygen scavengers undergo sacrificial oxidation during the cycling of lithium batteries. The resultant electrolytes improve long-term cycling and rate performance significantly.

Rechargeable Li metal batteries are currently limited by safety concerns, continuous electrolyte decomposition and rapid consumption of Li. These issues are mainly related to reactions occurring at the Li metal–liquid electrolyte interface. The formation of a passivation film (that is, a solid electrolyte interphase) determines ionic diffusion and the structural and morphological evolution of the Li metal electrode upon cycling. In this Review, we discuss spontaneous and operation-induced reactions at the Li metal–electrolyte interface from a corrosion science perspective. We highlight that the instantaneous formation of a thin protective film of corrosion products at the Li surface, which acts as a barrier to further chemical reactions with the electrolyte, precedes film reformation, which occurs during subsequent electrochemical stripping and plating of Li during battery operation. Finally, we discuss solutions to overcoming remaining challenges of Li metal batteries related to Li surface science, electrolyte chemistry, cell engineering and the intrinsic instability of the Li metal–electrolyte interface. Rechargeable Li metal batteries are currently limited by electrolyte decomposition and rapid Li consumption. Li plating and stripping greatly depend on the solid electrolyte interphase formed at the Li metal–liquid electrolyte interface. This Review discusses the reactions occurring at this interface from a corrosion science perspective, highlighting the requirements for an ideal passivation layer.

The lithium metal battery (LMB) is one of the most promising next-generation battery systems due to its ultrahigh energy density. However, problematic dendrite formation and low Coulombic efficiency (CE) greatly limit its practical application. Carbonate electrolyte solvents are still indispensable for the operation of LMBs using a transition metal oxide cathode. We determined the impact of different cyclic carbonates, which actively participate in the formation of the solid-electrolyte interface (SEI), on the stable cycling of LMBs using a nickel-rich layered cathode LiNi0.6Mn0.2Co0.2O2 (NMC622). The substitution of fluorine atoms in the cyclic carbonate profoundly enhances the stability of the lithium metal anode while fluoroalkyl and alkoxy substituents are detrimental. Cyclic carbonate trans-difluoroethylene carbonate (DFEC) was identified as a novel SEI enabler on the lithium metal anode, facilitating the formation of a protective SEI with relatively high lithium fluoride content. A Li/NMC622 cell utilizing DFEC electrolyte solvent as SEI enabler displayed a capacity retention larger than 82% after 400 cycles and an average CE of 99.95%. In contrast, the cycling retention after 400 cycles for a Li/NMC622 cell using monofluoroethylene carbonate was only 31% with an average CE of 99.73%. Other fluoroalkyl or alkoxy cyclic carbonates do not provide improved stabilization of the lithium metal anode over ethylene carbonate. The fundamental studies in this work provide critical insight for the further development of advanced electrolytes in LMBs.

From dictating the redox potential of electrolyte solvents to shaping the stability of solid-electrolyte interfaces, solvation plays a critical role in the electrochemistry of electrolytes. To efficiently design functional electrolytes for lithium batteries, it is particularly important to understand the relative solvating ability of each individual organic solvent, because most of the electrolyte systems are comprised of two or more electrolyte solvents. Using a newly developed internally referenced diffusion-ordered spectroscopy technique and diffusion coefficient–coordination ratio (D–α) analysis, we successfully constructed a solvating power series for common electrolyte solvents. We demonstrated the usefulness of this solvating power series in designing more reliable electrolyte system by selecting an appropriate fluorinated electrolyte solvent for a high-voltage lithium metal battery (LMB) as an example. For a methyl(2,2,2-trifluoroethyl)carbonate-based electrolyte, we identified fluoroethylene carbonate as a more desirable cyclic carbonate co-solvent than difluoroethylene carbonate for LMB due to its significantly higher ability to solvate lithium ions.

Relative solvating power, that is, the ratio of the coordination ratios between a solvent and the reference solvent, was used to probe the quantitative structure-activity relationship of electrolyte solvents and the lithium polysulfide (LiPS) dissolution in lithium-sulfur batteries. Internally referenced diffusion-ordered nuclear magnetic resonance spectroscopy (IR-DOSY) was used to determine the diffusion coefficient and coordination ratio, from which the relative solvating power can be easily measured. The higher the relative solvating power of an ethereal solvent, the more severe will be the LiPS dissolution and the lower the coulombic efficiency of the lithium-sulfur battery. A linear relationship was established between the logarithm of relative solvating power of a solvent and the degree of LiPS dissolution, rendering relative solvating power an important parameter in choosing the electrolyte solvent for lithium-sulfur batteries.

A novel methodology is reported on the use of internally referenced diffusion-ordered spectroscopy (IR-DOSY) in divulging the solution structure of lithium-ion battery electrolytes. Toluene was utilized as the internal reference for 1H-DOSY analysis due to its exceptionally low donor number and reasonable solubility in various electrolytes. With the introduction of the internal reference, the solvent coordination ratio of different species in the electrolytes can be easily determined by 1H-DOSY or 7Li-DOSY. This new technique was applied to different carbonate electrolytes, and the results were consistent with a Fourier transform infrared (FTIR) analysis. Compared to conventional vibrational spectroscopy, this IR-DOSY technique avoids the complicated deconvolution of the spectrum and allows determination of the solvent coordination ratio of different species in electrolyte systems with two or more organic solvents.

A new single-solvent electrolyte system comprising lithium bis(fluorosuflonyl) imide (LiFSI) and β-fluorinated sulfone (TFPMS) was designed to enable very stable long-term cycling of high-voltage lithium-ion batteries. Compared to other fluorinated solvents such as α-fluorinated sulfone (FMES) and fluorinated carbonate (FEMC), which are prone to reduction on the graphite anode, the LiFSI-TFPMS system displayed outstanding compatibility with graphite. While regular carbonate and sulfone from the LiFSI electrolyte system are compatible with the graphite anode, their high solvating power not only induces severe corrosion on the aluminum cathode current collector at high voltage, but also renders a low aggregation level at a normal salt concentration (about 1.0 M), resulting in the formation of an unstable solid-electrolyte interphase (SEI) on the graphite anode. Owing to the low solvating power of TFPMS, the aggregation level of the LiFSI-TFPMS system is relatively high even at normal salt concentration, which not only facilitates the formation of a robust SEI by the sacrificial decomposition of LiFSI, but also suppresses the aluminum corrosion of the LiFSI electrolyte system at high voltage. Together with the high intrinsic anodic stability of TFPMS, the superior cycling performance of graphite||LiNi0.6Co0.2Mn0.2O2 cells was achieved by employing the non-flammable LiFSI-TFPMS single-solvent electrolyte system.

We report acrylonitrile (AN) as an effective additive in carbonate-based electrolytes to enable uniform and dense lithium (Li) deposition and to improve the coulombic efficiency of Li metal anode. Our electrochemical, spectroscopic, and theoretical study reveal that AN is cathodically electropolymerized on the Li surface prior to the electrochemical decomposition of the electrolyte during Li deposition. The resultant polyacrylonitrile artificial solid electrolyte interphase enables uniform nucleation and growth of Li deposition with significantly reduced side reactions. The effectiveness of the AN additive is demonstrated in 0.4 Ah Li||LiNi0.6Mn0.2Co0.2O2 pouch cells (using 50-μm Li anode, 3 mAh cm−2 cathode areal capacity, and 4 g Ah−1 electrolyte) with excellent cycle stability under realistic charge-discharge condition.

To facilitate the massive commercialization of electric vehicles, it is imperative to enhance the energy density of the state-of-the-art lithium battery technology. [1] Lithium metal battery (LMB) is considered as a promising energy storage system because of the exceptionally low electrochemical potential and ultrahigh specific capacity of the lithium metal anode. [2-4] Yet, the extensive application of LMBs is still obstructed by the extreme reactivity of lithium metal, which leads to severe safety problems. [5-6] To tackle this issue, the re-design of the electrolyte is viewed as the most practical approach due to its cost effectiveness. [7-8] In this presentation, we successfully establish a solvation rule for the SEI enabler to achieve stable cycling of the lithium metal anode through a variety of electrochemical and solvation studies such as a Li/Cu cell test and IR-DOSY analysis. Fluoroethylene carbonate (FEC) was used as the exemplary solid-electrolyte interphase (SEI) enabler because it is the most common SEI enabler in the LMB electrolyte, and it is able to form a desirable SEI on the lithium metal surface. The results of all electrochemical studies point to the existence of a critical ratio between FEC and a co-solvent (e.g., DMC, EMC, and EA) for the stable cycling of LMBs. If the FEC/co-solvent ratio is lower than the critical ratio, not only will an unstable SEI be formed, but the SEI will also be gradually deteriorated by the significant decomposition of lithium complexes, with Li+ solvated solely by the co-solvent/non-SEI enabler. We discovered that the solvation number of FEC must be ≥ 1 to ensure the formation of a stable SEI, which mainly resulted from the sacrificial reduction of the SEI enabler and subsequent repair of the SEI to facilitate stable cycling of LMBs. This finding is crucial for the future development of functional electrolytes in that the ratio of FEC to co-solvent must exceed the critical volume/molar ratio for the solvation number of FEC to be ≥ 1. Reference: Schmuch, R.; Wagner, R.; Hörpel, G.; Placke, T.; Winter, M., Nature Energy 2018, 3, 267–278. Li, X.; Zheng, J.; Engelhard, M. H.; Mei, D.; Li, Q.; Jiao, S.; Liu, N.; Zhao, W.; Zhang, J. G.; Xu, W., ACS Applied Materials & Interfaces 2018, 10 (3), 2469-2479. Xu, W.; Wang, J.; Ding, F.; Chen, X.; Nasybulin, E.; Zhang, Y.; Zhang, J.-G., Energy Environ. Sci. 2014, 7 (2), 513– Fang, C.; Li, J.; Zhang, M.; Zhang, Y.; Yang, F.; Lee, J. Z.; Lee, M. H.; Alvarado, J.; Schroeder, M. A.; Yang, Y.; Lu, B.; Williams, N.; Ceja, M.; Yang, L.; Cai, M.; Gu, J.; Xu, K.; Wang, X.; Meng, Y. S., Nature 2019, 572, 511. Alvarado, J.; Schroeder, M. A.; Pollard, T. P.; Wang, X.; Lee, J. Z.; Zhang, M.; Wynn, T.; Ding, M.; Borodin, O.; Meng, Y. S.; Xu, K., Environ. Sci. 2019, 12, 780. Liang, X.; Pang, Q.; Kochetkov, I. R.; Sempere, M. S.; Huang, H.; Sun, X.; Nazar, L. F. Nature Energy 2017, 2 (9), 17119. Zheng, J.; Engelhard, M. H.; Mei, D.; Jiao, S.; Polzin, B. J.; Zhang, J.-G.; Xu, W. Nature Energy 2017, 2, 17012. Cao, X.; Ren, X.; Zou, L.; Engelhard, M. H.; Huang, W.; Wang, H.; Matthews, B. E.; Lee, H.; Niu, C.; Arey, B. W.; Cui, Y.; Wang, C.; Xiao, J.; Liu, J.; Xu, W.; Zhang, J.-G., Nature Energy 2019, 4, 796.

Understanding the decaying mechanism in lithium-ion batteries (LIBs) is critical to establishing a stable electrolyte system. Despite the advent of various novel electrolyte solvents designated for high-voltage LIBs, their working principles are not fully understood. Currently, oxidative decomposition of electrolytes is believed to be the major cause of capacity fade, and tremendous effort has been devoted to discovering a new electrolyte with enhanced anodic stability. However, the oxidative decomposition process cannot solely explain the rapid decay of some electrolyte systems with intrinsic high anodic stability when used with a high-nickel layered oxide cathode such as LiNi0.6Mn0.2Co0.2O2 (NMC622). In this report, a study of the quantitative structure-activity relationship was conducted to deepen the mechanistic understanding of the decay in high-voltage LIBs. The results obtained from the newly introduced molecular pair analysis and linear free-energy relationship (LFER) studies were highly consistent with the solvation-involved decaying mechanism in a high-nickel layered oxide cathode cycling at high voltage (> 4.5 V vs. Li/Li+). There was no evidence correlating the solvation ability of electrolyte solvents with the decay of a high-nickel layered oxide cathode cycling at a relatively low voltage (

A high performance lithium–sulfur (Li–S) battery comprising a symmetric fluorinated diethoxyethane electrolyte coupled with a fish-scale porous carbon/S composite electrode was demonstrated. 1,2-Bis(1,1,2,2-tetrafluoroethoxy)ethane (TFEE) was first studied as a new electrolyte solvent for Li–S chemistry. When co-mixed with 1,3-dioxolane (DOL), the DOL/TFEE electrolyte suppressed the polysulfide dissolution and shuttling reaction. When coupled with a fish-scale porous carbon/S composite electrode, the Li–S cell exhibited a significantly high capacity retention of 99.5% per cycle for 100 cycles, which is far superior to the reported numerous systems.

New fluorinated sulfones were synthesized and evaluated in high voltage lithium-ion batteries using a LiNi0.5Mn0.3Co0.2O2 (NMC532) cathode. Fluorinated sulfones with an α-trifluoromethyl group exhibit enhanced oxidation stability, reduced viscosity and superior separator wettability as compared to their non-fluorinated counterparts. Their improved performance in high voltage NMC532/graphite cells makes them promising high voltage electrolytes for next generation high voltage high energy lithium-ion batteries.

State-of-the-art lithium-ion batteries (LIBs) are based on a lithium transition metal oxide cathode, a graphite anode and a nonaqueous carbonate electrolyte. To further increase the energy and power density of LIBs, silicon anodes have been intensively explored due to their high theoretical capacity, low operation potential, and low cost. However, the main challenges for Si anode are the large volume change during lithiation/delithiation process and the instability of the solid-electrolyte-interphase associated with this process. Recently, significant progress has been achieved via advanced material fabrication technologies and rational electrolyte design in terms of improving the Coulombic efficiency and capacity retention. In this paper, new developments in advanced electrolyte and additive for LIBs with Si anode were systematically reviewed, and perspectives over future research were suggested.

Triethlylphosphite (TEP) and tris(2,2,2-trifluoroethyl) phosphite (TTFP) have been evaluated as electrolyte additives for high-voltage Li-ion battery cells using a Ni-rich layered cathode material LiNi0.5Co0.2Mn0.3O2 (NCM523) and the conventional carbonate electrolyte. The repeated charge/discharge cycling for cells containing 1 wt % of these additives was performed using an NCM523/graphite full cell operated at the voltage window from 3.0-4.6 V. During the initial charge process, these additives decompose on the cathode surface at a lower oxidation potential than the baseline electrolyte. Impedance spectroscopy and post-test analyses indicate the formation of protective coatings by both additives on the cathode surface that prevent oxidative breakdown of the electrolyte. However, only TTFP containing cells demonstrate the improved capacity retention and Coulombic efficiency. For TEP, the protective coating is also formed, but low Li(+) ion mobility through the interphase layer results in inferior performance. These observations are rationalized through the inhibition of electrocatalytic centers present on the cathode surface and the formation of organophosphate deposits isolating the cathode surface from the electrolyte. The difference between the two phosphites clearly originates in the different properties of the resulting phosphate coatings, which may be in Li(+) ion conductivity through such materials.

This talk will describe a new, high-performance Pb-based nanocomposite anode material for lithium-ion batteries. A unique nanocomposite structure of core-shell Pb@PbO nanoparticles and carbon matrix was able to be obtained by a simple high energy ball milling method using the low-cost PbO and carbon black starting materilas. The electrochemical performance tests showed the excellent reversible capacity (ca. 600 mAh/g) and cycle stability (92% retention at 100th cycle), which are the best performances ever reported for Pb-based anodes in the literature. Synchrotron X-ray diffraction and absorption techniques revealed the detailed lithium storage mechanisms that can be highlihghted with the unexpectedly wide reversible Pb redox range (between Pb2+ and Pb4-) and the evolution of Zintl-type LiyPb structures during the electrochemical lithium reaction. Our results provide new insights into the lithium storage mechanism of the Pb-based materials and their potential as low-cost, high-performance anodes.

The cost of batteries is becoming as important as the energy density for large-scale electrochemical energy storage application. Sodium manganese-rich layered oxides (P-type and O-type) are thus one of the promising cathode materials for rechargeable batteries because of the relatively low cost of manganese and sodium. The O-type cathodes are more promising for practical application owing to their high sodium-ion stoichiometry. However, most of these materials suffer from rapid capacity decay during high-voltage cycling. Herein, we used synchrotron X-ray probes coupled with electrochemical techniques to disclose the structural evolution of α-NaMnO2 during solid-state synthesis and electrochemical cycling. During high-voltage cycling, a substantial increase of interfacial microstrain from both stacking faults and Mn cation migration was found to pin down this material’s layered structures and simultaneously block the diffusion pathways of Na+, thus leading to the performance degradation. The findings prese...

Organic sulfone compounds have been widely used as high-voltage electrolytes for lithium-ion batteries for decades. However, owing to the complexity of the synthesis of new sulfones, only a few commercially available sulfones have been studied. In this paper, we report the synthesis of new sulfone compounds with various substituent groups and the impact of the substituent group on the oxidation stability of sulfones. Electrochemical floating tests using a 5 V LiNi0.5Mn1.5O4 spinel cathode and density functional theory calculations showed that the cyclopentyl-substituted sulfone McPS suffered from oxidation instability, starting from 4.9 V versus Li+/Li, as observed by the large leakage currents. On the other hand, the isopropyl-substituted sulfone MiPS and tetramethylene substituted sulfone TMS showed much improved oxidation stability under identical testing conditions. The substitution structure of the sulfone plays a significant role in the determination of its oxidative stability and should first be considered for the development of new sulfone-based electrolytes for high-voltage, high-energy lithium-ion batteries.

A new class of electrolyte additives based on cyclic fluorinated phosphate esters was rationally designed and identified as being able to stabilize the surface of a LiNi0.5Mn0.3Co0.2O2 (NMC532) cathode when cycled at potentials higher than 4.6 V vs Li+/Li. Cyclic fluorinated phosphates were designed to incorporate functionalities of various existing additives to maximize their utilization. The synthesis and characterization of these new additives are described and their electrochemical performance in a NMC532/graphite cell cycled between 4.6 and 3.0 V are investigated. With 1.0 wt % 2-(2,2,2-trifluoroethoxy)-1,3,2-dioxaphospholane 2-oxide (TFEOP) in the conventional electrolyte the NMC532/graphite cell exhibited much improved capacity retention compared to that without any additive. The additive is believed to form a passivation layer on the surface of the cathode via a sacrificial polymerization reaction as evidenced by X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonsance (NMR) analysis...

Four different chiral diamino diethers synthesized from N-isopropyl valinol or N-isopropyl alaninol were lithiated with n-butyllithium in tetrahydrofuran or diethyl ether. Crystal structures of the dilithiated diamino diethers were determined by X-ray diffraction. Three dilithiated diamino diethers including (2S,2'S)-1,1'-(butane-1,4-diylbis(oxy))bis(N-isopropylpropan-2-amine) 7, (2S,2'S)-1,1'-(pentane-1,5-diylbis(oxy))bis(N-isopropylpropan-2-amine) 8, and (2S,2'S)-1,1'-(heptane-1,7-diylbis(oxy))bis(N-isopropyl-3-methylbutan-2-amine) 9 are dimers, whereas dilithiated (2S,2'S)-1,1'-(pentane-1,5-diylbis(oxy))bis(N-isopropyl-3-methylbutan-2-amine) 10 is a monomer. The lithium atoms in all crystal structures adopt a nonequivalent coordination protocol and exist in two different environments in which one of the lithium atoms is tetra-coordinated while the other one is tri-coordinated. The solution structures of the dilithiated diamino diethers are also characterized by a variety of NMR experiments including diffusion-ordered NMR spectroscopy (DOSY) with diffusion coefficient-formula (D-FW) weight correlation analyses and other one- and two-dimensional NMR techniques.

Crystal structure determination of lithiated N-methylaniline with a variety of ligands, including tetrahydrofuran, methyltetrahydrofuran, trans-2,5-dimethyltetrahydrofuran, dimethoxyethane, tetrahydropyran and N,N-diethylpropionamide, reveals a common Li–N–Li–N four-membered-ring dimeric structure motif. A progression of solvation from tetrasolvated dimer (PhNMeLi·S2)2 through trisolvated dimer to disolvated dimer (PhNMeLi·S)2 was observed by increasing the steric hindrance of the ligand. Solid-state structures of several other lithium N-alkylanilides solvated by tetrahydrofuan are also reported. When the methyl group of N-methylaniline is replaced by an isopropyl or a phenyl group, trisolvated monomers are formed instead of dimers. Interestingly, the solid-state structure of lithiated N-isobutylaniline in tetrahydrofuran is a trisolvated dimer while that of lithium N-neopentylanilide is a disolvated dimer.

We report the development of isotopic-labeled (13)C diffusion-ordered NMR spectroscopy (DOSY) NMR with diffusion coefficient-formula weight (D-FW) analysis and its application in characterizing the aggregation state of methyllithium aggregates and complexes with several widely used diamines. Commercially available (13)C-labeled benzene and several easily synthesized (13)C-labeled compounds using (13)C-labeled iodomethane as the isotopic source are developed as internal references for diffusion-formula weight analysis (D-FW). The technique greatly expands the applicability of DOSY D-FW analysis to a much wider variety of compounds because of isotopic labeling. These results reveal that methyllithium exists as a tetrasolvated tetramer in diethyl ether and exclusively as bis-solvated dimers with chelating diamines.

The crystal structure of a mixed aggregate containing lithiated (S)-N-ethyl-3-methyl-1-(triisopropylsilyloxy)butan-2-amine derived from (S)-valinol and cyclopentyllithium is determined by X-ray diffraction. The mixed aggregate adopts a ladder structure in the solid state. The ladder-type mixed aggregate is also the major species in a toluene-d8 solution containing an approximately 1:1 molar ratio of the lithiated chiral amide to cyclopentyllithium. A variety of NMR experiments including diffusion-ordered NMR spectroscopy (DOSY) with diffusion coefficient-formula (D-FW) weight correlation analyses and other one- and two-dimensional NMR techniques allowed us to characterize the complex in solution. Solution state structures of the mixed aggregates of n-butyl, sec-butyllithium, isopropyllithium with lithiated (S)-N-ethyl-3-methyl-1-(triisopropylsilyloxy)butan-2-amine are also reported. Identical dimeric, ladder-type, mixed aggregates are the major species at a stoichiometric ratio of 1:1 lithium chiral amide to alkyllithium in toluene-d8 solution for all of the different alkyllithium reagents.

The solid-state structures of unsolvated, hexameric cyclopentyllithium and tetrameric cyclopentyllithium tetrahydrofuran solvate were determined by single-crystal X-ray diffraction. Cyclopentyllithium easily crystallized in hydrocarbon solvents. Solution-state structural analyses of cyclopentyllithium and cyclopentyllithium-tetrahydrofuran complexes in toluene-d8 were also carried out by diffusion-ordered NMR spectroscopy with diffusion coefficient-formula weight correlation analyses and other one- and two-dimensional NMR techniques. The solution-state studies suggest that unsolvated cyclopentyllithium exists as hexamer and tetramer equilibrating with each other. Upon solvation with tetrahydrofuran, cyclopentyllithium exists mostly as a tetrahydrofuran tetrasolvated tetramer.

Solution-state characterization of sec-butyllithium in [D8]toluene was conducted by utilizing a variety of NMR spectroscopic experiments including diffusion-ordered NMR spectroscopy (DOSY) with diffusion-coefficient–formula-weight correlation analyses and other one- and two-dimensional NMR spectroscopic techniques. These results suggest that sec-butyllithium exists primarily as a tetramer/hexamer mixture in hydrocarbon solvents. However, the presence of roughly 2–5 % (by mol ratio) lithium sec-butoxide in both commercially available and laboratory-synthesized sec-butyllithium solutions due to the reaction with adventitious dioxygen will lead to the formation of a 1:5 sec-butoxide/sec-butyllithium mixed hexamer. A sec-butoxide/sec-butyllithium mixed octamer will emerge when the proportion of butoxide to butyllithium increases.

Developing a high-voltage enabling electrolyte is extremely critical for the success of the next generation high-energy density lithium-ion battery especially for electric vehicles. Material scientists have developed new cathode materials with improved specific capacity[1,2] and operating voltage (5 V vs. Li+/Li).[3,4] However, designed for a 4V-class lithium-ion chemistry, the conventional electrolyte suffers from oxidation instability on the charge cathode/electrolyte interface at high charging voltages which leads to severe transition metal (TM) dissolution and rapid capacity fading. The voltage instability of electrolyte becomes the bottleneck for the extensive application of the high voltage cathode materials. Thus, new electrolytes with elevated voltage stability has been widely explored.[5-8] Here we report a new class of fluorinated electrolytes comprising novel fluorinated sulfones including ((trifluoromethyl)sulfonyl)ethane (FMES), 1-((trifluoromethyl)sulfonyl)propane (FMPS) and 2-((trifluoromethyl)sulfonyl)propane (FMIS).[9] These compounds have been synthesized via new synthetic routes and evaluated as electrolyte materials under high voltage operation. The results indicate that sulfones with an α-trifluoromethyl group possess enhanced oxidative potential and reduced viscosity as compared to their non-fluorinated counterparts. The α-fluorinated sulfones also show enhanced wetting ability with polyolefin separator. A facile synthesis method for a reported sulfone 1,1,1-trifluoro-3-(methylsulfonyl)propane (FPMS) was also developed. With the new reaction method, large quantity of material was obtained and comprehensive electrochemical properties of FPMS as high voltage electrolyte was evaluated. Unlike the α-fluorinated sulfones, the γ-fluorinated sulfones resemble the property of the non-fluorinated ones. References: [1] Nyten, A.; Abouimrance, A.; Armand, M.; Gustafsson, T.; Thomas, J. O. Electrochem. Commun., 2005, 7, 156-160. [2] Ellis, B. L.; Makahnoul, R. M.; Makimura, Y.; Toghill, K.; Nazar, L. F., Nat. Mater., 2007, 6, 749-753. [3] Hu, M.; Pang, X.; Zhou, Z. J., Power Sources, 2013, 237, 229-242. [4] Santhanam, R.; Rambabu, B. J., Power Sources, 2010, 195, 5442-5451. [5] He, M.; Su, C. C.; Feng, Z.; Zeng, L.; Wu, T.; Bedzyk, M. J.; Fenter, P.; Wang, Y.; Zhang, Z., Adv. Energy Mater., 2017, 7, 1700109. [6] He, M.; Su, C. C.; Peebles, C.; Feng, F.; Connell, J. G.; Liao, C.; Wang, Y.; Shkrob, I. A.; Zhang, Z., ACS Appl. Mater. Interface, 2016, 8(18), 11450-11458. [7] Kunduraciz, M.; Amatucci, G.G., J. Electrochem. Soc., 2006, 153, A1345-A1352. [8] Wolfenstine, J.; Aleen, J., J. Power Sources, 2005, 142, 389-390. [9] Su, C. C.; He, M.; Redfern, P. C.; Curtiss, L. A.; Shkrob, I. A.; Zhang, Z., Energy Environ. Sci., 2017, 10, 900-904. Figure 1

Lithium-ion batteries have been widely applied in portable electronics due to its high energy density. [1] It is essential to improve the state-of-the-art LIBs, which are far from their maximum theoretical energy density, in order to power up electric vehicles. [2] Researchers have shown that the physiochemical properties, such as conductivity, electrochemical window and ability to form solid electrolyte interphase (SEI), depend on the solution structures of the electrolyte. [3] Thus, it is crucial to determine the solution structures of the electrolyte and the relative solvating ability of various solvents for designing advanced electrolyte systems. Novel method utilizing specific 2D-NMR technique was successfully developed to probe the solvation state of different electrolyte solvents. The result obtained by this technique agreed with the measurement by vibrational spectroscopy. Most importantly, unlike the vibrational spectroscopy measurement, this method can be easily applied to electrolyte system with two or more electrolyte solvents. For example, it is not impossible to determine the solvation state of a ternary-solvent electrolyte comprising EC, EMC and DMC by vibrational spectroscopy due to the severe overlapping of peaks. However, the solvation state can be easily determined with the new method developed. Therefore, this new technique provides an important experimental tool in examining the solvation state of different electrolyte solvents. Application of this newly developed technique in revealing the electrochemical performance of various electrolytes on Li-ion batteries will also be discussed. References: [1] (a) Tarascon, J.-M.; Armand, M., Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2001, 414, 359-367. (b) Armand, M.; Tarascon, J.-M., Building Better Batteries. Nature 2008, 451, 652-657. [2] Goodenough, J. B.; Park, K.-S., The Li-Ion Rechargeable Battery: A Perspective. Journal of the American Chemical Society 2013, 135, 1167-1176. [3] (a) Seo, D. M.; Reininger, S.; Kutcher, M.; Redmond, K.; Euler, W. B.; Lucht, B. L., Role of Mixed Solvation and Ion Pairing in the Solution Structure of Lithium Ion Battery Electrolytes. The Journal of Physical Chemistry C 2015, 119 (25), 14038-14046. (b) Xu, K., Electrolytes and interphases in Li-ion batteries and beyond. Chemical reviews 2014, 114 (23), 11503-618.

1,4-Dimethoxybenzene derivatives are materials of choice for use as catholytes in non-aqueous redox flow batteries, as they exhibit high open-circuit potentials and excellent electrochemical reversibility. However, chemical stability of these materials in their oxidized form needs to be improved. Disubstitution in the arene ring is used to suppress parasitic reactions of their radical cations, but this does not fully prevent ring-addition reactions. By incorporating bicyclic substitutions and ether chains into the dialkoxybenzenes, a novel catholyte molecule, 9,10-bis(2-methoxyethoxy)-1,2,3,4,5,6,7,8-octahydro-1,4:5,8-dimethanenoanthracene (BODMA), is obtained and exhibits greater solubility and superior chemical stability in the charged state. A hybrid flow cell containing BODMA is operated for 150 charge–discharge cycles with a minimal loss of capacity.

A high voltage LiNi0.5Mn0.3Co0.2O2/graphite cell with a fluorinated electrolyte formulation 1.0 m LiPF6 fluoroethylene carbonate/bis(2,2,2-trifluoroethyl) carbonate is reported and its electrochemical performance is evaluated at cell voltage of 4.6 V. Comparing with its nonfluorinated electrolyte counterpart, the reported fluorinated one shows much improved Coulombic efficiency and capacity retention when a higher cut-off voltage (4.6 V) is applied. Scanning electron microscopy/energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy data clearly demonstrate the superior oxidative stability of the new electrolyte. The structural stability of the bulk cathode materials cycled with different electrolytes is extensively studied by X-ray absorption near edge structure and X-ray diffraction.

The solution structures of three mixed aggregates dissolved in toluene-d8 consisting of the lithiated amides derived from (S)-N-isopropyl-1-((triisopropylsilyl)oxy)propan-2-amine, (R)-N-(1-phenyl-2-((triisopropylsilyl)oxy)ethyl)propan-2-amine, or (S)-N-isobutyl-3-methyl-1-((triisopropylsilyl)oxy)butan-2-amine and n-butyllithium are characterized by various NMR experiments including diffusion-ordered NMR spectroscopy with diffusion coefficient-formula weight correlation analyses (D-FW) and other one- and two-dimensional NMR techniques. We report that steric hindrance of R1 and R2 groups of the chiral lithium amide controls the aggregation state of the mixed aggregates. With a less hindered R2 group, lithium (S)-N-isopropyl-1-((triisopropylsilyl)oxy)propan-2-amide forms mostly a 2:2 ladder-type mixed aggregate with n-butyllithium. Increase of steric hindrance of the R1 and R2 groups suppresses the formation of the 2:2 mixed aggregate and promotes formation of a 2:1 mixed aggregate. We observe that lithium (S)-N-isobutyl-3-methyl-1-((triisopropylsilyl)oxy)butan-2-amide forms both a 2:2 mixed aggregate and a 2:1 mixed trimer with n-butyllithium. Further increase in the steric hindrance of R1 and R2 groups results in the formation of only 2:1 mixed aggregate as observed with lithium (R)-N-(1-phenyl-2-((triisopropylsilyl)oxy)ethyl)propan-2-amide.

The solid state structure of lithiated (S)-N(1),N(1)-bis(2-methoxyethyl)-N(2),3-dimethylbutane-1,2-diamine, which is a chiral amide base synthesized from (S)-valine was determined by single-crystal X-ray diffraction. The complex in solution state is also characterized by a variety of NMR experiments including diffusion-ordered NMR spectroscopy (DOSY) with diffusion coefficient-formula weight correlation analyses and other one- and two-dimensional NMR techniques by dissolving the crystal in toluene-d8. The crystallography and NMR results suggest that the chiral amide is dimeric in both solid and solution states.

The lithium-ion battery is an ideal power source for electrified vehicles due to its long cycle life and high energy and power density. To further increase the energy density, the general approach is to use cathode materials with high operating voltages (5 V vs. Li+/Li) and high specific capacity (250 mAh/g). Tremendous attention has been paid to raising the specific capacity and the discharge voltage of the cathode materials. For example, two-Li+-ion material such as silicate Li2FeSiO4 and fluorophosphate Li2FePO4F has been proposed and investigated as a high capacity cathode. The theoretical capacity of these two materials is high, 323 and 292 mAh/g, respectively. There also have been many studies on expanding the operating voltage. Cathode materials with a potential of more than 5 V have been proposed, for example, olivine-type LiNiPO4 and LiCoPO4, and normal spinel-type LiNi0.5Mn1.5O4 and LiCoMnO4. However, these cathodes require an electrolyte with high oxidation stability to deliver the full capacity. Okada et al. proposed Li2CoPO4F as both a high-capacity and a high-voltage cathode material. However, its practical capacity is 120 mAh/g, and its energy density showed no advantage over LiCoPO4 because of electrolyte decomposition above 5 V. The conventional lithium-ion battery employs organic carbonate esters as the electrolyte solvent, in particular, mixtures of ethylene carbonate (EC) with dimethyl carbonate (DMC), diethyl carbonate (DEC), and/or ethyl methyl carbonate (EMC) dissolved in LiPF6 salt. This electrolyte continuously decomposes above 4.5 V vs. Li+/Li, limiting its application to a cathode chemistry that delivers capacity at a high charging voltage. Therefore, the demand for a high-voltage electrolyte has become a high priority for the development of lithium-ion batteries with high energy density. Several studies have been dedicated to the development of high voltage electrolytes such as sulfones, ionic liquids, and dinitriles, as well as electrolyte additives that stabilize the charged cathode surface to afford a reversible Li+-intercalation chemistry in the coveted 5-V region. These electrolyte materials can provide high anodic stability, but they suffer from their intrinsic high viscosity, low dielectric constant, and low conductivity. More importantly, they do not form a solid-electrolyte interphase (SEI) on carbonaceous anode material. Moreover, it is a major challenge to develop an electrolyte additive for these new electrolytes that will provide the SEI formation required for extended cycling performance, especially under abuse conditions. Although an additive for cathode SEI formation was reported to be able to suppress the reactivity of the charged electrode and electrolyte, a superior electrolyte system comprising solvents with intrinsic anodic stability is the ultimate solution for high voltage electrolytes. In this presentation, new electrolyte based on fluorinated carbonate solvents were designed, syntehsized and evaluated with high voltage cathode materials at elevated temperature. The theoretically high anodic stability of these new electrolytes was supported by electrochemical evaluation results using LiNi0.5Mn1.5O4 spinel cathodes and LiNi0.5Co0.2Mn0.3O2 cathodes. Fluorinated carbonate appears to be a suitable electrolyte candidate for transition metal oxide cathodes at high voltage.